The present invention relates to a differential input A/D converter which converts the potential difference between two analog input signals from an analog value to a digital value.
In a differential input A/D converter which converts the potential difference between two analog input signals from an analog value to a digital value, the potential difference between two analog input voltages needs to be converted into a digital value at the same time. In addition, in order to perform fine control by using a control device such as microcontroller on the basis of a converted digital value, conversion with higher accuracy is required.
The conventional differential input A/D converter shown in FIG. 6 converts first and second analog input signals 1 and 2 into a single-end signal 7 by using an analog subtracter 101. After the single-end signal 7 is sampled by a sample/hold circuit 102, the sampled value is converted into a digital value by a successive approximation type A/D converter 103. This signal is then output as a conversion result 6. As shown in FIG. 7, the analog subtracter 101 is comprised of three operational amplifiers 111 to 113 and resistors 104 to 110.
FIG. 8 shows another conventional differential input A/D converter. Referring to FIG. 8, the conventional differential input A/D converter converts a first analog input signal 1 into a digital value by using a first A/D converter 114 and outputs it as a first conversion result 11. At the same time, the differential input A/D converter converts a second analog input signal 2 into a digital value by using a second A/D converter 115 and outputs it as a second conversion result 12. The differential input A/D converter then calculates the difference between the first and second conversion results 11 and 12 by using a digital subtracter 116, and outputs the difference as a differential conversion result 14.
In the differential input A/D converter shown in FIG. 6, the three operational amplifiers 111 to 113 constituting the analog subtracter 101 have errors such as offsets. These errors are superimposed to become a conversion error, and hence the error in the differential input A/D converter increases. In addition, in order to prevent an increase in error due to the operational amplifiers 111 to 113, the differential input A/D converter must incorporate high-precision operational amplifiers 111 to 113. This however causes an increase in chip area.
In addition, only when the first analog input signal 1 higher in voltage than the second analog input signal 2, the potential difference can be A/D-converted. If, however, the first analog input signal 1 is lower in voltage than the second analog input signal 2, the conversion result becomes zero.
Letting ADCR1 be the value of the first conversion result 11 in the differential input A/D converter shown in FIG. 8, and ADCR2 be the value of the second conversion result 12, a value ADCR of the differential conversion result 14 can be given by
ADCR=ADCR1xe2x88x92ADCR2xe2x80x83xe2x80x83(1) 
In addition, letting Vin1 be the voltage of the first analog input signal 1, Vin2 be the voltage of the second analog input signal 2, Verr1 be the conversion error in the A/D converter 114, Verr2 be the conversion error in the A/D converter 115, V(ADCR1) be the function for which the first conversion result 11 is converted into an analog voltage, and V(ADCR2) be the function for which the second conversion result 12 is converted into an analog voltage, the voltages Vin1 and Vin2 are given by
Vin1=V(ADCR1)+Verr1xe2x80x83xe2x80x83(2) 
Vin2=V(ADCR2)+Verr2xe2x80x83xe2x80x83(3) 
Letting V(ADCR) be the function for which the differential conversion result 14 is converted into an analog voltage, the differential conversion result 14 obtained by the differential input A/D converter can be generally given by
Vin1xe2x88x92Vin2=V(ADCR)+Verrxe2x80x83xe2x80x83(4) 
Substitutions of equations (2) and (3) into equation (4) yield equation (5):                                                                                           V                  ⁡                                      (                    ADCR                    )                                                  +                Verr                            =                                                {                                                            V                      ⁡                                              (                        ADCR1                        )                                                              +                    Verr1                                    }                                -                                  {                                                            V                      ⁡                                              (                        ADCR2                        )                                                              +                    Verr2                                    }                                                                                                        =                                                V                  ⁡                                      (                    ADCR1                    )                                                  -                                  V                  ⁡                                      (                    ADCR2                    )                                                  +                Verr1                -                Verr2                                                                        (        5        )            
The conversion error in the differential conversion result 14 therefore becomes (Verr1xe2x88x92Verr2). Since the conversion errors Verr1 and Verr2 are independent of each other, the error in the differential conversion result 14 is equal to the value obtained by superimposing the errors in the two A/D converters 114 and 115. Therefore, in the differential input A/D converter shown in FIG. 8, if the errors in the two A/D converters 114 and 115 are similar, the conversion error becomes almost double, at maximum, that when one A/D converter is used.
With regard to quantization errors which A/D converters theoretically have, in particular, a quantization error of xe2x88x92xc2xd to +xc2xd LSB in the A/D converter 114 and a quantization error of xe2x88x92xc2xd to +xc2xd LSB in the A/D converter 115 are added together. As a consequence, the differential input A/D converter has a quantization error of xe2x88x921.0 to +1.0 LSB. In other words, if the resolution of the two A/D converters 114 and 115 is n bits, the resolution of the differential input A/D converter constituted by the A/D converter 114 and 115 becomes substantially (nxe2x88x921) bits, which is smaller than n bits by one bit.
It is an object of the present invention to provide a high-precision differential input A/D converter without increasing the precision of A/D converters and analog subtracters.
In order to achieve the above object, according to the present invention, there is provided a differential input A/D converter which converts a potential difference between two analog input signals from an analog value to a digital value, comprising first A/D conversion means for outputting a first conversion result obtained by A/D-converting a first analog input signal, second A/D conversion means for outputting a second conversion result obtained by A/D-converting a second analog input signal, and digital subtraction means for outputting a differential conversion result obtained by subtracting the second conversion result output from the second A/D conversion means from the first conversion result output from the first A/D conversion means, wherein the first A/D conversion means A/D-converts a difference between an analog value obtained from an output of the second A/D conversion means and the second analog input signal output from the second A/D conversion means and superimposes the A/D conversion result on the first conversion result.